Recycling melamine synthesis offgas to urea synthesis

ABSTRACT

OFFGAS CONSISTING ESSENTIALLY OF AMMONIA AND CARBON DIOXIDE IS FORMED IN THE SYNTHEIS OF MELAMINE FROM UREA. THIS IS ABSORBED IN TWO STAGES IN WATER OR AN AQUEOUS SOLUTION OF AMMONIUM CARBAMATE OR UREA FOR RETURN TO UREA SYNTHESIS. IN THE FIRST STAGE THE GAS IS TREATED WITH AN AMOUNT OF ABSORBENT SOLUTION WHICH IS NOT SUFFICIENT TO ABSORB THE WHOLE OF THE GAS. THE REMAINING OFFGAS AND THE SOLUTION FORMED ARE THEN COMPRESSED SEPARATELY FROM ONE ANOTHER TO THE PRESSURE WHICH IS SUFFICIENT FOR THE ABSORPITON OF THE REMAINING OFFGAS INTO THE SOLUTION AND THE OFFGAS IS THEN ABSORBED. THE SOLUTION THUS FORMED IS RETURNED TO THE HIGH PRESSURE SECTION OF THE SYNTHESIS PLANT.

Jan. 2 1973 E. HILLEINBRAND 3,708,536

RECYCLING MELAMINE SYNTHESIS OFFGAS TO UREA SYNTHESIS Filed July 23,1969 l v 2 I I IF I 3 I L I9 I 1 IO 12 I I I I e x i I C02 t l3 7 Q NH3ENGELBERT HILLENBRAND ATT'YS United States Patent 3,708,536 RECYCLINGMELAMINE SYNTHESIS OFFGAS T0 UREA SYNTHESIS Engelbert Hillenbrand,Ludwigshafen, Germany, assignor 1 to Badische Anilin- & Soda-FabrikAktiengesellschaft,

Ludwigshafen (Rhine), Germany Filed July 23, 1969, Ser. No. 844,089Claims priority, application Germany, July 25, 1968, P 17 70 969.2 Int.Cl. C07c 127/00 US. Cl. 260-555 A 4 Claims ABSTRACT OF THE DISCLOSUREOlfgas consisting essentially of ammonia and carbon dioxide is formed inthe synthesis of melamine from urea. This is absorbed in two stages inwater or an aqueous solution of ammonium carbamate or urea for return tourea synthesis. In the first stage the gas is treated with an amount ofabsorbent solution which is not sufficient to absorb the whole of thegas. The remaining offgas and the solution formed are then compressedseparately from one another to the pressure which is sufiicient for theabsorption of the remaining oifgas into the solution and the offgas isthen absorbed. The solution thus formed is returned to the high pressuresection of the synthesis plant.

In the synthesis of melamine from urea carried out at atmospheric orsuperatmospheric pressure according to the equation:

ammonia and carbon dioxide are formed as byproducts in the ratiorequired for the synthesis of urea. For this reason the urea andmelamine syntheses have been combined with each other in such a way thatthe melamine synthesis offgas is utilized by supplying it to ureasynthesis.

Various proposals have already been made for utilizing the melaminesynthesis oifgas for the synthesis of urea but these relate mostly tothe synthesis of melamine carried out at superatmospheric pressure.

According to one prior art method, the melamine oifgas obtained at apressure of from 150 to 300 kg./cm. and a temperature of from 300 to 400C. is introduced direct into the urea reactor which is provided withcooling means for removing the heat introduced with the hot gases.

According to another prior art method (U.S. patent specification No.3,239,522) the melamine synthesis oifgas is combined with the olfgasobtained by expanding the urea melt, the combined otfgas stream isscrubbed with a solution containing ammonium carbamate and the solutionis recycled to urea synthesis.

Generally the equilibrium relationships within and between theindividual decompression or absorption stages of a urea recyclesynthesis process are so correlated that appreciable amount of makeupammonia and carbon dioxide cannot be supplied in any stages withoutadditional solvent as for example water, aqueous ammonium carbamatesolution or urea-containing ammonium carbamate solution being provided.While it is true that recycle melamine offgas replaces part of thestarting materials of urea synthesis, the water required to absorb thisoffgas has an unfavorable effect on urea synthesis and on the layout ofthe plant.

There are no significant problems in absorbing oifgas which is atsuperatmospheric pressure because in this case naturally the amount ofliquid necessary for absorption of the gas is fairly small.

This amount of water is considerably greater however when offgas fromthe melamine synthesis which is at atmospheric or only slightly elevatedpressure is recycled to urea synthesis by absorption in water. Inprinciple it would be possible to compress this ofigas prior to itsabsorption and to absorb it in an appropriately smaller amount ofsolvent at superatmospheric pressure. This procedure has thedisadvantage however that a large amount of energy is required for thecompression of the Whole of the offgas.

With regard to the recirculation of ofi'gas at atmospheric pressure tourea synthesis it is known that this olfgas may be absorbed in water oran aqueous solution containing ammonium carbamate which aftercompression is subjected to desorption at superatmospheric pressure. Thegas mixture of ammonia and carbon dioxide thus obtained is then absorbedin a fairly small amount of solvent and supplied to urea synthesis. Themethod also has the disadvantage of a high energy consumption due to thehigh heat requirement for the desorption of the gas under pressure.

It is an object of the present invention to return the olfgas underatmospheric or only slightly elevated pressure obtained in melaminesynthesis to urea synthesis with the smallest possible amount of waterand with the smallest possible consumption of energy.

We have found that this object is achieved by absorbing a part of theoffgas, e.g. about 26 to 35% by weight in a first absorption stage in anamount of water (which may contain ammonium carbamate and/or urea) whichis in sufiicient to absorb the whole of the offgas, at the pressure atwhich it is obtained, and then compressing the residual oflgas and theaqueous phase separately from each other to a pressure sufficient toabsorb the residual offgas in the aqueous phase, absorbing the residualofigas at this pressure in a second absorption stage and introducing thesolution obtained into the high pressure zone of the urea synthesis.

The process according to this invention may be carried out by treatingthe whole of the melamine oifgas in the first stage at the pressure atwhich it is obtained with an amount of solvent which is sufiicient todissolve the whole of the olfgas in the amount of solvent used at thehigher pressure of the second absorption stage. The undissolved gas andthe aqueous phase obtained are then compressed separately to thepressure of the second stage and at this pressure the residual gas isabsorbed in the aqueous phase.

It is however also possible to divide the olfgas, before it is washedwith the solvent, one portion being equivalent to the amount which willbe completely absorbed by the amount of solvent used at the lowerpressure, i.e. at the pressure at which the olfgas is obtained, whilethe remaining portion of the olfgas is not treated with the absorbentuntil it has been compressed and passed into the second stage. This hasthe advantage that the portion not absorbed at the lower pressure onlycomes into contact once with the solvent.

The pressure for absorption of the offgas in the first stage isdetermined by the pressure at which the melamine ofigas is obtained.This is from about 1 to 5 atmospheres absolute.

The pressure to be maintained in the second stage may vary within widelimits. It is advantageous to choose for this stage a pressureequivalent to the pressure prevailing in the decompression stage of ureasynthesis in which the major portion of the ammonium carbamate which hasnot been converted into urea is decomposed. In this stage the pressureis from about 10 to 30 atm. abs.

Depending on the amount of melamine offgas to be recycled, its ratio ofammoniazcarbon dioxide and the amount of any Water content, anddepending on the pressure chosen in the second absorption stage, theamount of water is calculated which is necessary in order to absorb thewhole of the melamine oifgas according to the process of this invention.

Since the stages for the absorption of the melamine oifgas are similaras regards their pressures to the stages present in the urea recyclemethod in which the offgas of the urea synthesis is absorbed again, theotfgas of the urea synthesis and of the melamine synthesis which areequivalent in pressure are combined and absorbed together in a preferredembodiment of the process according to this invention.

An embodiment of the process according to this invention will now bedescribed with reference to the drawings:

1 denotes a reactor in which ammonia and carbon dioxide are convertedinto urea by a conventional method. The melt is withdrawn from thereactor and freed from ammonium carbamate which has not been convertedinto urea in a decompression stage 2 at a pressure of 10 to 30atmospheres absolute and in a decompression stage 3 at a pressure ofabout 1 to atm. abs. The offgas streams from the two decompressionstages, which consist essentially of ammonia, carbon dioxide and water,are passed respectively into absorbers 4 and 5 in which they areabsorbed into a solvent. The solvent may be for example vapor condensateif the solution coming from decompression stage 3 is evaporated in(evaporator method) or the mother liquor freed from urea crystals mayserve as solvent if urea is crystallized therefrom (crystallizationmethod). The solvent first passes through absorber 5 and then iscompressed by means of a pump to the higher pressure of the absorber 4.A concentrated ammonium carbamate solution having a water content ofabout 20% is conveyed from absorber 4 through a pressure pump 7 into thereactor 1. When urea synthesis is carried out with a large excess ofammonia, pure ammonia may additionally be withdrawn from absorber 4,condensed in a cooler 8 and returned to a reservoir 9 whence it passesagain into reactor 1. The urea solution coming from the lastdecompression stage, as already stated above, is either evaporated at 10or subjected to crystallization. The anhydrous urea in the form of amelt, crystalls or prills is introduced at least in part into a melamineplant 12 and converted therein into melamine. The urea not required formelamine synthesis is withdrawn at 141. The ofrgas from melaminesynthesis passes through line 13 into an absorber 14 into which afurther portion of the solvent is introduced through line 6. Melamine isWithdrawn through line 19. The amount of solvent is so regulated that itis sufficient to absorb the whole of the melamine ofigas at the higherpressure of the following absorption stage 15. The offgas not absorbedin absorber 14 is raised to the pressure of absorber 15 in a compressor16, while the aqueous phase is compressed by means of a pump 17 andintroduced into absorber 15. Here the unabsorbed gas is wholly taken upby the aqueous phase. The concentrated carbamate solution having acontent of about of water which leaves absorber 15 is conveyed by meansof a pressure pump 18 into reactor 1.

Since the absorbers 4 and 15 and the pressure pumps 7 and 18 operateunder approximately the same conditions, it is possible to combine thetwo absorbers and to condense the otfgas of urea synthesis and ofmelamine synthesis in an aqueous phase in a single stage and to returnit by means of a single pump to the urea reactor.

The invention is illustrated by the following examples.

EXAMPLE 1 A gas mixture consisting of 370 parts by weight of ammonia and370 parts by Weight of carbon dioxide is introduced into absorber 14(cf. drawing) at a pressure of 1.2 atm. abs. and scrubbed at atemperature of 50 C. with parts by weight of water. 26% by weight of thegas is absorbed by the water. The unabsorbed gas has its pressure raisedby means of compressor 16 to that which prevails in the second absorber15 and is then introduced into the same. The aqueous phase withdrawnfrom the first column 14 is similarly compressed to 21 atm. abs. by thepump 17 and is passed into the column 15 for the remainder of the gasmixture to be absorbed.

If the gas mixture is to be absorbed in water in a single absorber at apressure of 1.2 atm. abs. and a temperature of 50 C., the amount ofwater required would be 710 parts by weight.

EXAMPLE 2 The gas mixture, under otherwise the same conditions as inExample 1, is treated with water in column 14 but at a pressure of 2.5atm. abs. 35% by weight of the gas mixture is absorbed. The remainder ofthe gas mixture is absorbed in column 15 at 21 atm. abs. as described inExample 1.

I claim:

1. A process for recycling offgas, which is formed in the synthesis ofmelamine from urea at atmospheric or slightly elevated pressure andconsisting essentially of ammonia and carbon dioxide, to a high pressureurea synthesis zone for the reaction of carbon dioxide and ammonia toammonium carbamate and conversion of the ammonium carbamate into urea,which process comprises: absorbing about 26 to 35% by weight of saidoffgas in a first absorption stage at 1 to 5 atmospheres in an amount ofwater, which may contain ammonium carbamate and/or urea, which isinsuflicient to absorb the Whole of the offgas; and then compressing theresidual ofigas and the aqueous phase from said first stage separatelyfrom each other to a higher pressure of from 10 to 30 atmospheres whichis sufficient to absorb the residual otfgas in the aqueous phase; whollyabsorbing said residual oifgas from said first stage at this higherpressure in a second absorption stage; and introducing the solution thusobtained into the high pressure zone of the urea synthesis.

2. A process as claimed in claim I]. wherein the oifgas from melaminesynthesis is combined with the otfgas from urea synthesis which is at asimilar pressure to the oifgas from melamine synthesis and both oifgasesare absorbed together.

3. A process as claimed in claim 1 wherein said water contains at leastone of the compounds ammonium carbamate and urea.

4. A process as claimed in claim 1 wherein the first absorption stage ismaintained at the same pressure as the oifgas obtained from the melaminesynthesis.

References Cited .UNITED STATES PATENTS 3,544,628 12/1970 Hsu 260-555 A3,414,571 12/1968 Haines et al 260-2497 A 3,492,302 1/1970 Abe et al.2.60555 A 3,503,970 3/1970 Kanai et a1. 260555 A BERNARD HELFIN, PrimaryExaminer M. W. GLYNN, Assistant Examiner U.S. Cl. X.R. 260-2497

